Include OpenJ9 JDK 21 in CI build plan #307

Job	Run time
build (ubuntu-latest, 11, temurin)	2m 56s
site (ubuntu-latest, 11, temurin)	1m 21s
build-all (ubuntu-latest, 8, temurin)	2m 38s
build-all (ubuntu-latest, 8, zulu)	2m 27s
build-all (ubuntu-latest, 8, adopt-openj9)	2m 39s
build-all (ubuntu-latest, 11, zulu)	3m 3s
build-all (ubuntu-latest, 11, adopt-openj9)	3m 31s
build-all (ubuntu-latest, 17, temurin)	2m 58s
build-all (ubuntu-latest, 17, zulu)	2m 51s
build-all (ubuntu-latest, 17, adopt-openj9)	3m 45s
build-all (ubuntu-latest, 21, temurin)	2m 53s
build-all (ubuntu-latest, 21, zulu)	3m 10s
build-all (ubuntu-latest, 21, adopt-openj9)	3m 53s
build-all (windows-latest, 8, temurin)	4m 13s
build-all (windows-latest, 11, temurin)	4m 32s
build-all (windows-latest, 17, temurin)	4m 20s
build-all (windows-latest, 21, temurin)	4m 40s
build-all (macOS-latest, 8, temurin)	4m 14s
build-all (macOS-latest, 11, temurin)	3m 28s
build-all (macOS-latest, 17, temurin)	6m 36s
build-all (macOS-latest, 21, temurin)	4m 7s
	1h 14m 15s

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